498 Proceedings of the Royal Irish Academy. 



reports, for instance, that, during the first tkree montlis lie used lis 

 apparatus, he found an error = + 0^-3, but that he, by constant prac- 

 tice, reduced it to + 0''11, at which rate it continued for more than 

 six months.'^ He, therefore, recommends young astronomers to be 

 " educated" by practising with the artificial apparatus, as large errors, 

 eventually appearing in this way, may be diminished, when the ob- 

 server has remarked their existence. Kaiser has been of the same 

 opinion, and has always kept his apparatus ready for work. They 

 have, however, been very little used,f and it is still doubtful whether 

 such an " education" is of much use. Of coui'se, the apparatus ought 

 to be very carefully adjusted during such exercises, and experienced 

 astronomers (who, it is to be supposed, are not in want of any such. 

 "education") ought to observe with them at the same time. It 

 is not improbable that the personal error may be changed by practice, 

 in the eye-and-ear method, as well as in the chronographic method ; 

 indeed, my own experience, however limited, makes me inclined 

 to think so. We know that a perfectly unexperienced observer's 

 error, within a veiy short interval of time, even on one and the same 

 evening, has changed considerably ;% and besides, an observer has 

 often himself felt that he pei-formed a certain act (for instance, the 

 touching of the key) too early or too late, and when he has found 

 this out, it is comparatively easy to correct the fault. § But it seems 

 to be but very seldom that such a case happens, and until experience 

 has taught us otherwise, it must be considered as very doubtful whe- 

 ther an "education," such as the one above mentioned, is of much use. 

 It is certainly an unpleasant cuxumstance that personal equations 

 and errors are often variable, as one may fear that the value used 

 for the reduction of a series of observations is not the right one, if it is 

 not found exactly at the time when these observations were made, 

 which is not always possible ; for instance, at determinations of longi- 

 tude. The invention of the chronographic method was about twenty- 

 five years ago hailed with pleasure, as it was expected that the per- 

 sonal error by this method should become more constant, as well 

 as smaller. II But these expectations have only partly been realized, 



* -Annales de 1' Observatoire de Pans, Mem., t. viii., p. 171. 



t Annalen der Stemwarte in Leiden, ii., p. 26. 



+ In tlie Memoii-es des Asfronomes de Poulkova, t. ii. : Exped. Chronom. 

 de 1845, p. 52, there is mentioned a Lieutenant Alexandi-o^^v of the topographic corps, 

 whose error (found by comparisons with the other experienced members of the 

 expedition) varied enormously. It seemed as if A. only for a few hours kept the 

 same custom in obseiving, but that his eiTor changed (often 0^-3 or O^'i) when 

 there was a larger interval between the different observations. It was at fii'st 

 expected that A. by practice could become more regular, but tliis was not the case. 



^ So, for instance, when the chronographic method was introduced in Leyden, 

 (Kaiser, Yerslagen, ii., p. 232). "We shall afterwards see how an excellent 

 observ'er, Schonfeld, in the course of years, has changed his way of estimating 

 transits of nebulas, when it had been remai'ked that his right ascensions were 

 too small. 



II W. C. Bond, in the Report of the British Assoc, 1851. 



